The precise location of the replication terminus on the chromosome of Escherichia coli is being determined. Cells containing prophage P2 located at different sites in the chromosome are treated so that P2 initiates replication of the chromosome at or near the P2 site, and the rate, direction, and extent of this replication is determined. DNA-DNA hybridization is used to study this replication, and the DNA's used in the assays are obtained from Proteus mirabilis/ E. coli hybrid cells containing various E. coli episomes, or from various bacteriophage lambda specialized transducing phage. Preliminary experiments have demonstrated that the replication terminus affects the P2 derived replication forks that reach the region of the chromosome. Another situation that is being exploited to study termination is the use of cells in which deletions and additions have occurred in various parts of the chromosome and the normal symmetry of the replicating chromosome has been altered. Autoradiography is also being used to study termination in these cells, and also in the P2 lysogens. Autoradiographical procedures will also be used to determine the amount of DNA present in the terminus region of the chromosome that appears to contain no genetic markers. BIBLIOGRAPHIC REFERENCES: Maglothin, P.D. and P.L. Kuempel. Direction of Chromosome Replication in Escherichia coli dna A Mutant Integratively Suppressed by Prophage P2. Bacteriol. Proc., 1976.